In times of increasing resource scarcity and climate change resulting from an increase in the concentration of atmospheric carbon dioxide, renewable energy—especially energy production with PV systems—is growing in importance. To achieve economical energy generation with minimal use of cables, a number of PV modules, each with a number of PV cells, are connected serially in a so-called string, thereby generating a high DC voltage. This DC voltage is transformed in an inverter into a grid-compatible AC voltage so that the DC power generated by the string can be fed into the grid as AC power and used at other locations.
By connecting PV modules in series as a string, DC voltages are achieved that can exceed maximum values allowed for safety reasons. For example, in the USA, in such systems only DC voltages lower than 600V with respect to ground potential are allowed. This limits the maximum DC voltage of a string to 1200 V, as long as it is ensured that the string potential is distributed symmetrically relative to the ground potential. This can be achieved by grounding at the string midpoint, for example. However, such a grounding limits the flexibility of the configuration of the power generation system, in particular when used with transformerless inverters.
Inverter topologies exist that can guarantee a distribution of the potential of the connected string to ground potential during operation within the allowed range even without midpoint grounding, avoiding the limitations described above. However, measures must be taken here to ensure that potentials at all points on the string remain within the range allowed when the inverter is not in operation.
Document U.S. Pat. No. 6,593,520 presents a process for decoupling a faulty string from an inverter. First, the connection of the string to the inverter is decoupled, then the faulty string is divided into a number of substrings, wherein the voltage of each substring is chosen in such a way that as a result of the division, the string is put in a safe state.
Document U.S. Pat. No. 7,768,751 describes a power generation system that detects a ground fault in the conductors of the DC side of a string using a ground fault detector. When a ground fault is detected, the string is decoupled from the inverter and divided into substrings, wherein the substrings are grounded using auxiliary switches.
Another method for detecting a ground fault is presented in document DE102006022686.